It is well known that the direction of magnetization of a magnetic material can be reversed or switched by impressing an external magnetic field of opposite polarity on the material. Typically, such magnetic material may be placed upon a non-magnetic substrate and then divided or separated into a grid-like pattern to create a display composed of individual magnetic post elements. If the magnetic post elements and the non-magnetic substrate are structured from materials that are transparent to electromagnetic radiation and the magnetic material exhibits the Faraday effect of rotating polarized light, it is possible to create a magneto-optic display by passing polarized light through the transparent element and then through an analyzer. Because reversing the direction of magnetization of the magnetic material also reverses its associated direction of Faraday rotation, it is possible to effectively switch the optical properties of the individual display elements on and off by reversing the direction of magnetization of each post element in response to an externally switched magnetic field. When electric conductors are placed within the separations between the post elements, electric current may be passed through these conductors to generate the magnetic flux about each conductor. When this flux is concentrated at the point of coincidence of two conductors, the concentrated magnetic flux may be used to reverse the direction of magnetization of the post element located at that point to accomplish the switching of the element.
Reversal of the direction of magnetization of the magnetic material within the post element, in response to the imposition of an external magnetic field, occurs at the area which has the lowest value of anisotropy field within that magnetic material. At this low anisotropy field area, a magnetic domain wall, which separates adjacent regions of material where each region has an opposite direction of magnetization, is generated and then propagated through the rest of the magnetic material to complete the switching of the post element. Thus, switching of a post element is composed of an initiating step and a saturation step. To initiate switching of the post element, a magnetic field which exceeds the effective anisotropy field of that area of the post element with the lowest anisotropy must be generated. To complete switching through magnetic field saturation, the domain wall must spread to the farthest reaches of the post element. This will only occur if the magnetic field imposed upon the most distant portion of the post element is higher than the saturation field of the magnetic material forming the element.
Typically, magneto-optic devices are fabricated using bismuth doped liquid phase epitaxial garnet films. The presence of bismuth improves the Faraday rotation of the garnet film which is the principal optical property utilized in such devices. Increasing the bismuth concentration within the garnet film raises both the Faraday rotation and the anisotropy constant of the film. High anisotropy constants yield high anisotropy fields, which, in turn, result in the requirement of higher magnetic switching field thresholds to change the direction of magnetization of the garnet films. Conversely, higher anisotropy constants lead to lower magnetic saturation fields. Thus, it is apparent that desired optical, magnetic, and switching properties of such films cannot be achieved independently of one another and that optimizing one property may harm another property.
It would be desirable to have a magneto-optic device in which these opposing requirements for desired properties could all be optimized. A copending patent application entitled "Altering The Switching Threshold Of A Magnetic Material", filed Nov. 12, 1981, Ser. No. 320,819, now abandoned, which is assigned to the common assignee of the present invention, discloses a method and device for reducing the switching field required to reverse the direction of magnetization in which a region of low anisotropy material is implanted within a high anisotropy material forming a post element. However, there is a practical limitation to this method of switching threshold reduction because increasing the Faraday rotation of such devices through increased bismuth concentrations will also increase the switching thresholds. Thus, the prior art fails to disclose a magneto-optic device in which the desired optical, magnetic, and switching properties can be independently and simultaneously optimized.
Examples of other prior art devices for switching the direction of magnetization of magnetic post elements upon a non-magnetic substrate may be found in Letters U.S. Pat. No. 3,715,736, issued Feb. 6, 1973, and in Letters U.S. Pat. No. 4,114,191, issued Sept. 12, 1978. Another prior art patent which uses an external magnetic field and coincident current selection of post elements in a magneto-optic display may be found in British Pat. No. 1,180,334, published Apr. 5, 1968.